Soot particles consist mainly of elemental carbon (C), which may contain components of high and low volatility in its composition. Without a catalyst and under normal pressure, the C particles burn at temperatures above 500° C. in the presence of oxygen. Hydrocarbons, sulfates, oxides, polycyclic aromatic hydrocarbons, etc., may be deposited on the nanostructures of the carbon particles.
The percentage of the volatile components in the exhaust gas varies depending on the load state of combustion equipment or an internal combustion engine. Volatile particle components can be oxidized in the exhaust gas stream more easily than highly coked particle agglomerates. Emissions of diesel engines are referred to as hazardous substances of Class I and as hazardous and cancer-causing substances in the Threshold Limit Values List (MAK-Liste), see Appendix 1 to the Ordinance on Hazardous Substances (Gefahrstoffverordnung).
Emissions from diesel engines are subject to regulations such as the Technical Rules for Hazardous Substances [Technische Regeln für Gefahrstoffe (TRGS)], in which they are classified as carcinogenic. Exhaust gas limit values of <0.15 g/kWh are currently in force in Europe. The limits will be reduced to <0.05 g/kWh in the European Union according to the “EURO IV Ordinance” beginning from 2005. To meet the limit values, a device with a catalytically active carrier matrix with at least one free passage for the exhaust gases is necessary. The provision of many passages through the matrix increases the conversion capacity in the gas stream, which is also called conversion rate Kr.
The device must withstand continuous thermodynamic and mechanical alternating stresses and shock as well as permanent chemical loads. To burn particles in the exhaust gas stream, time must be available in order for energy to be able to act on the particles. Since the soot particles are entrained in the exhaust gas stream at a speed greater than 10 m/sec, and the device must not have a large volume, only a short time is left for the conversion. In order to introduce the needed energy into the particles in a short time and not to compromise the internal combustion engine by an unacceptable back pressure during the process, the following conditions must be met:    1. Open (0.1–0.5 mm) labyrinth channels through the integral carrier matrix monolith,    2. large geometric area of the carrier matrix,    3. large number of contacts (impact) of the particles with the catalyst,    4. selective (multistep) catalytic treatment of the particles and gases,    5. conversion in the temperature window of 180–500° C.,    6. thermal stability of the matrix during exothermal spontaneous reactions,    7. no deposition of ashes at operating temperatures below 800° C.,    8. low back pressure (below 150 mbar),    9. avoidance of the generation of NOx,    10. avoidance of excess NO2,; and    11. avoidance of CO emission.
In compliance with the EURO IV/V exhaust gas limit values the passage channels through the device are dimensioned such that they are many times larger than the largest diameter of the largest particle agglomerate ever occurring. Deposition or accumulation of carbon particles cannot thus occur either in the highly loaded state or in the nonloaded operating state of a combustion equipment. The exhaust gas stream is consequently not compromised by the device as a consequence of the dimensioning of the passage channels, regardless of the load state of the combustion equipment. An increase in the back pressure caused by the accumulation of Carbon particles in the device is ruled out when a controlled regeneration is performed if the engine begins to operate above a defined back pressure. A controlled regeneration can be performed by the control engineering system of the combustion equipment itself or by external measures (e.g., auxiliary heater, electric heater, cavity resonator, dipole excitation, high-voltage discharge, dielectrically pulsed low-temperature plasma).
The use of centrifuges, cyclones, particle filters, and oxidation catalysts for separating particles from exhaust gas streams has been known. Small, lightweight and inexpensive devices that make possible the continuous conversion of respirable carbon particles are needed in small, mobile units. Such devices have not become known for small and mobile units (5–120 kW) and have not been used for such units, either. Contrary to this, catalytic filters and oxidation catalysts are used.
According to their physical principle, particle filters tend to exhaust their filter capacity, and the back pressure, which increases due to clogged filter surfaces, opposes the flowthrough of the exhaust gas, especially the separation of respirable particles. This circumstance is unfavorable for the internal combustion engine arranged upstream of the “aftertreatment.” In case of a combination of an internal combustion engine with a filter arranged downstream, the internal combustion engine cannot be operated, e.g., at full load when the filter is already exhausted. This may lead to damage to the internal combustion engine.
Additives for filter burn-off, which burn the solid particles in the exhaust gas continuously or intermittently and in a satisfactory manner, are ecologically objectionable. The regeneration of exhaust gas filter cartridges is complicated in terms of control engineering and expensive in terms of handling. Nonregenerable filters, i.e., “disposable filters,” represent an ecological problem. Such exhaust gas cleaning units cannot therefore be considered for wide use based on ecological considerations, especially because the filters are exhausted before the end of the normal life cycle and are to be considered to be special waste in case of disposal.